Roofing
Underlayment
A water-resistant layer installed directly over the roof decking and under the finish roofing material to provide secondary protection against water infiltration.
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In the Field
Felt (#15 or #30) has been replaced on most projects by synthetic underlayments that are lighter, stronger, and more tear-resistant. Always check manufacturer requirements — installing the wrong underlayment can void a shingle warranty.
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Rule of Thumb
Minimum IRC requirement is #15 felt or equivalent synthetic on slopes of 4:12 and greater. Slopes between 2:12 and 4:12 require double-layer underlayment with specific lapping requirements.